This invention relates to electronic devices, and more particularly to a packaged high frequency (or radio frequency (RF)) power semiconductor device with either a direct-bonded metal substrate or a plastic package, or both.
The use of semiconductor transistors for RF applications has increased dramatically over the years as heat dissipation, size, dependability, and other improved characteristics have made such transistors well-suited for many applications. Such semiconductor transistors are used in electrical circuits which operates at 1 MHz or greater, typically at 25 MHz or even 500 MHz or greater. In some of such electrical circuits, the transistors are part of the power conversion circuit and are required to act as the switching element, in frequencies greater than 1 MHz, as in advance switch mode power supplies, resonant converters, DC to DC converters, and the like. For these applications, a lower cost RF type package is preferably used to provide a competitive solution in comparison to the lower frequency prior art applications.
These RF power transistors are typically housed in packages (xe2x80x9cRF packagesxe2x80x9d) to form RF power devices. The RF packages provide mechanical support and a means for dissipating heat for integrated circuits. Mechanical support is necessary to facilitate handling and prevent breakage of the integrated circuit during installation or operation of the device. Heat dissipating characteristics of the RF packages are particularly important for RF power devices which have high power outputs. RE devices typically output from 1 to 1,000 Watts during their normal operations.
The packages for such high power transistors must be capable of dissipating the relatively high quantities of heat generated by the transistor to maintain the transistor at a suitable operating temperature, i.e., a temperature which will not adversely affect the operating characteristics of the transistor. Heat decreases the performance and operating life of the transistor. Moreover, heat generated at the collector-base junction can create a temperature buildup which may eventually cause the device to short-circuit. Another important characteristic for such packages is that they provide for good electrical grounding of the transistor with a minimum of parasitic inductance.
A semiconductor die of the RF power device may be mounted on a thermally conductive pad which acts as an internal heatsink (i.e., a component provided as a part of the power device) and dissipates the heat generated by the power device. In order to prevent electrical losses, however, the thermally conductive pad must also be electrically insulating. Hence, a thermally conducting, electrically insulating material is used for the mounting pad. Beryllia (beryllium oxidexe2x80x94BeO) and alumina (aluminum oxidexe2x80x94Al2O3) are commonly used materials for the mounting pad.
Of the two, beryllia is used more extensively due to its superior thermal conductivity over alumina. However, BeO is highly toxic material, and extreme care must be exercised in handling it. Beryllia can be hazardous to the human respiratory system when in the powder form, as the BeO pad is machined or polished. Proper equipment and safeguards are needed to insure that it is handled safely and that improper contact with humans is avoided. In fact, the United States military has prohibited the use of BeO except under exceptional circumstances. Therefore, among other improvements, it would be desirable to eliminate or reduce usage of Beryllia in fabricating RF power devices.
The invention provides a packaged power semiconductor device having electrical isolation between the potential of a die inside the package and a metal back side of the package. A power semiconductor die is mounted on a direct-bonded copper (xe2x80x9cDBCxe2x80x9d) substrate. The die may be mounted using solder or other die-attach means. A lead of the package is also soldered to the DBC substrate. In some embodiments, all leads are soldered to connecting pads on the DBC substrate.
In one embodiment, a radio frequency power device includes a substrate including a first conductive layer, a second dielectric layer, and a third conductive layer. The first conductive layer is bonded to the second dielectric layer, and the second dielectric layer is bonded to the third conductive layer. The first and third conductive layers are electrically isolated from each other. A semiconductor die is bonded to the first conductive layer of the substrate. A plastic package encloses and protects the semiconductor die. A plurality of leads extend outwardly from the plastic package. The leads have blade-like shapes.
A further understanding of the nature and advantages of the present invention may be realized by reference to the remaining portions of the specification and the drawings.